Thursday, December 20, 2007
They’ve been busy – feeding off oak leaf surfaces as larvae, wolfing down entire leaves as red-headed worms, morphing into tannish-brown moths, fluttering around in courtship, and laying clusters of whitish eggs on neighborhood oak trees.
“If you listen carefully, you can hear the worms chewing on the leaves,” says local arborist Rob Thompson, who sits on the Monterey Fire Safe Council. “It’s kind of a crackling sound.”
Oak moth outbreaks tend to occur every five to 10 years, and super-dry 2007 was one of those years. Evergreen coast live oaks weakened by back-to-back droughts have taken the hardest hit. The parasitic pest, which usually manages two breeding cycles annually, made it through three generations this year – a veritable moth explosion. The amount of damage varies from tree to tree, ranging from barely-noticeable leaf thinning to near total balding.
“Everybody’s on edge, and understandably so,” Thompson says. “People see their tree, which is normally green throughout the year, and suddenly it’s losing its leaves, and they don’t know exactly what the cause is.”
Landowners may assume their balding oaks are dead. But infested trees usually exhibit Lazarus-style stamina: While they may look homely for a few seasons, they tend to bounce back after the winter rains begin.
“Nine times out of 10 this year, when you see a coast oak that appears to be in rapid decline, it’s attributed to the California oak worm,” Thompson says, using the oak moth’s other name. “It’s more of an aesthetic issue. It’s rarely devastating to the health of the tree.”
The moths even give back: Their high-nutrient droppings, called frass, act as fertilizer. “If people don’t rake it away, it will actually revitalize these urban soils to a certain degree,” he says.
Owners of denuded trees may think they have to cut them down. But thanks to local regulations, most infested oaks dodge the axe. Permits are typically required for tree removals, though regulations vary from city to city and in different parts of the county. The approval process usually involves a visit from a certified arborist.
“We definitely had an increase in the numbers of [tree removal] requests, because people thought the trees were dead,” says Monterey City Forester Robert Reid. “But we didn’t necessarily approve them. We counseled [landowners] and encouraged them to retain their trees.”
Despite the outbreak, the oak moth’s fire was stolen by an even more notorious Lepidoptera: the light brown apple moth, an indiscriminate eater whose invasion prompted the state to spray sex-disrupting pheromones over Monterey and Santa Cruz counties this fall. The LBAM is smaller than the oak moth but sports a similar mottled wing design. While both coast oaks and oak moths are native to California, LBAMs hail from Australia.
“Because of the press on all the apple moth stuff, people were confusing the oak moth for the apple moth,” Reid says.
It’s also easy to mistake oak moth infestation for sudden oak death (SOD), a fatal and highly contagious fungal infection. Both the moth and the fungus defoliate coast live oaks, though only SOD targets tan oaks. “It’s pretty much impossible to tell by looking,” says Kerri Frangioso, a UC Davis research ecologist and SOD expert.
There are a few key differences between the two afflictions. SOD thrives in foggy places, such as Big Sur and some canyons in the lower Carmel Valley, but it hasn’t become a big problem in urban areas of the Peninsula. Oak moths, on the other hand, were widespread in the sunnier regions of Carmel, Monterey and the Highway 68 corridor this year.
Fog may make conditions less hospitabe to moths, Thompson says – which could explain why the city of Pacific Grove received fewer tree removal permit applications this year. “We don’t have a lot of oak moth in town,” says Celia Perez Martinez, PG Public Works supervisor. “It might be our little microclimate here – we’re socked in fog.”
The most important distinction: By the time SOD-infected trees lose their leaves, they’re dry and dead, while most trees defoliated by oak moths are alive. That makes all the difference in terms of fire risk.
“The oak moth doesn’t kill the tree unless it’s really stressed, so it’s able to defend itself more from a wildfire,” says Jan Bray, forester for the state fire protection department and Monterey Fire Safe Council member. “Sudden oak death girdles the tree, then it suddenly dies. After a year or so, those dead trees start falling over and add to the fuels, especially ladder fuels. They really increase the fire danger.”
Thompson stresses the importance of hiring a certified arborist to diagnose a denuded oak. If it’s infested with oak moths, patience may be the best medicine. Pesticides are sometimes justified for historic or high profile oaks, he says, but the moths are so ubiquitous that it’s an expensive and often futile battle.
“It’s important that people know that oak worm is not detrimental to the health of trees,” he says. “In most cases, the tree will recover.”